1. Field of the Invention
This invention relates in general to data communication systems, and more particularly, to a method and apparatus for suppression of jitter in data communication systems.
2. Description of Related Art
Waiting time jitter, or low frequency jitter, or wander, is well known phenomenon in data transmission systems. Generally, low frequency jitter occurs in pulse-stuff modulated digital circuits. Pulse-stuff modulation has been used to maintain synchronization in data communication systems which selectively insert pulses into a digital data frame. The output of a pulse-stuff modulation often contains low frequency jitter. The detailed description of this type of low frequency jitter can be found in the following references: 1) U.S. Pat. No. 5,539,785, entitled xe2x80x9cJITTER/WANDER REDUCTION CIRCUIT FOR PULSE-STUFFED, SYNCHRONIZED DIGITAL COMMUNICATION,xe2x80x9d issued to Burch et al., on Jul. 23, 1996; 2) an article, entitled xe2x80x9cA COMPREHENSIVE ANALYSIS OF STUFF THRESHOLD MODULATION USED IN CLOCK-RATE ADAPTION SCHEMES,xe2x80x9d authored by Saman S. Abeysekera et al., published by IEEE Trans. On Communications, vol. 46, no. 8, August 1998; 3) an article, entitled xe2x80x9cANALYSIS OF TECHNIQUES FOR THE REDUCTION OF JITTER CAUSED BY SONET POINTER ADJUSTMENTSxe2x80x9d, authored by Richard G. Kusyk, et al., published by IEEE Trans. On Communications, vol. 42, no. 2/3/4, February/March/April 1994; and 4) an article, entitled xe2x80x9cJITTER ANALYSIS OF A DOUBLE MODULATED THRESHOLD PULSE STUFFING SYNCHRONIZER,xe2x80x9d authored by Gianfranco L. Pierobon, et al., published by IEEE Trans. On Communications, vol. 39, no. 4, April 1991.
Techniques for reducing low frequency jitter are discussed in two of the above references: 1) U.S. Pat. No. 5,539,785; and 2) the article by Saman S. Abeysekera et al.
However, the technique discussed in U.S. Pat. No. 5,539,785 requires special hardware and non-standard methods for reducing the jitter. The technique requires coordination between a transmitter and a receiver, i.e. both sides of a data communication channel. The technique discussed in the article by Saman S. Abeysekera et al. shows an improved method called a xe2x80x9cthreshold modulated stuffing methodxe2x80x9d. This method eliminates low frequency jitter at a transmitter and does not require interaction between a transmitter and a receiver, i.e. the jitter is eliminated at the transmitter and is not seen at the receiver. However, the xe2x80x9cthreshold modulated stuffingxe2x80x9d method does not provide a good jitter suppression over wider operating limits as shown in FIG. 4.
Typically, in a data communication system, a transmit data rate may be different from a receive data rate. The transmit data is assembled into a transmit frame that carries the transmit data and the other overhead bits as needed in the system, for example, for maintenance, signaling, synchronization, etc. A frame generally has a variable length. For example, a frame may be a xe2x80x9cshortxe2x80x9d frame with a nominal frame size minus a variable, e.g. delta, or a frame may be a xe2x80x9clongxe2x80x9d frame with a nominal frame size plus a variable, e.g. delta. This may vary from system to system. For example, another system may have three frame sizes: a nominal frame size, a xe2x80x9cshortxe2x80x9d frame size, and a xe2x80x9clongxe2x80x9d frame size. Generally, a framer decides what type of frame to send at a transmitter depending on the level of a transmit data buffer. For instant, if the buffer tends to be over-filling, indicating that the transmit data is arriving slightly faster than the outgoing frame rate, then the framer would tend to send more xe2x80x9cshortxe2x80x9d frames than xe2x80x9clongxe2x80x9d frames, thereby effectively speeding up the communication channel data rate slightly and thus emptying the buffer. Likewise, if the buffer tends to be under-filling, indicating that the transmit data is arriving slightly slower than the outgoing frame rate, then the framer would tend to send more xe2x80x9clongxe2x80x9d frames than xe2x80x9cshortxe2x80x9d frames, thereby effectively slowing down the communication channel data rate slightly and allowing the transmit data buffer to re-fill.
In a xe2x80x9cnominalxe2x80x9d case where the transmit rate is exactly matched to a nominal transmission rate, the framer may send alternating xe2x80x9cshortxe2x80x9d and xe2x80x9clongxe2x80x9d frames. Alternatively, in a system that could transmit frames with nominal frame lengths, then the framer may send frames with nominal frame lengths. However, if a very slight frequency offset is present between the transmit data rate and the nominal transmission rate, then after some arbitrarily long period of time (depending on the frequency offset), the framer would inject an additional xe2x80x9cshortxe2x80x9d or xe2x80x9clongxe2x80x9d frame into the alternating long/short pattern. This extra injection occurs at a steady rate, thereby resulting in a very low frequency jitter.
In another case, for instance, if the nominal transmission rate happens to be exactly at some ratio of long/short frame patterns, then the framer may send a fixed pattern of, e.g. long long short . . . long long short . . . over and over. However, if the nominal transmission rate is very near the ratio, but not exactly at the ratio, then occasionally the framer injects an additional xe2x80x9clongxe2x80x9d or xe2x80x9cshortxe2x80x9d frame, thereby causing a very low frequency jitter.
In the existing systems, threshold modulated stuff/delete scheme was proposed to randomize the threshold modulation with some type of pattern, with the desired result that the pattern will modulate the lower frequency jitter to a higher frequency jitter that can be removed readily at a later point by a low pass filter. However, this threshold modulation is not reliable as jitter patterns can always be found that can defeat a modulation waveform.
It is with respect to these and other considerations that the present invention has been made.
In accordance with the present invention, the above and other problems are solved by providing a jitter suppression circuit to suppress or eliminate waiting time jitter or low frequency jitter at a transmitter end of a data transmission system.
In one embodiment of the present invention, a jitter suppression circuit in a data transmission system includes a phase detector circuit to determine a plurality of phase errors between sync pulses of a Digital Subscribe Line (DSL) and sync pulses of a reference line; and an adapted phase error offset circuit, coupled to the phase detector circuit, to generate a plurality of phase error offsets and adaptively offset the plurality of phase error offsets.
Further in one embodiment of the present invention, a jitter suppression method includes steps of detecting a phase error, computing a phase error offset, and filtering out the phase error offset.
Accordingly, the present invention allows the phase errors between the sync pulses of a Digital Subscribe Line (DSL) and the sync pulses of a reference line to be filtered out before inputting to a stuff/delete slicer which generates a plurality of stuff/delete signals for a framer. Waiting time jitter or low frequency jitter is thus eliminated and/or suppressed. The framer can utilize the stuff/delete signals to determine what type of data frames to send to a communication channel of the data transmission system.